Transdermal absorption preparation has many advantages in that the first pass effect in the liver or gastrointestinal tract can be avoided, a stable blood concentration can be maintained during adhesion since the drug is absorbed from the skin over a long period, administration can be discontinued when side effects are expressed and the like. However, since many drugs show low skin permeability, there are not many drugs actually formulated into transdermal absorption preparations, and a technique for improving transdermal absorbability of a drug has been desired.
Various methods have heretofore been considered to improve transdermal absorbability of a drug. As one of them, an attempt has been made to appropriately design a solution used for dissolving a drug to improve transdermal absorbability of the drug. For example, WO2009/066457 teaches that an external preparation composition comprising a fatty acid ionic liquid as an active ingredient improves transdermal absorbability of the drug. To be specific, in WO2009/066457, a fatty acid having 5-20 carbon atoms, an organic amine having 4-12 carbon atoms, ethanol and isopropanol, which are lower alcohols, and propylene glycol, which is a polyvalent alcohol, are used for external preparation compositions. Moreover, WO2000/53226 shows that a composition containing a higher alcohol having 8-12 carbon atoms and a polyvalent alcohol improves transdermal absorbability of a drug.
However, according to the study of the present inventors, such compositions that allegedly improve transdermal absorbability of conventional drugs fail to provide stable transdermal absorbability since volatilization of solvent changes the composition, or fail to provide a transdermal absorption preparation that expresses an expected pharmacological effect since skin permeation amount of drug cannot be sufficiently increased, or show a transdermal absorption enhancing effect only for a particular drug, and none of them showed a sufficiently superior absorption enhancing effect and broad utility. To markedly improve transdermal absorbability of a drug, therefore, a more drastic improvement of the composition of this kind of compositions is necessary.